Previous   |    Next

doi:10.2204/iodp.proc.340.203.2015

Results

A total of 29 holoplanktonic gastropod species within 16 genera and unidentified gymnosome veliger shells (Fig. F2) were identified in sediments offshore of Montserrat (Figs. F3, F4, F5, F6, F7, F8). Of the identified species, 16 are thecosome (shelled) pteropods, 1 is a gymnosome (shell-less) pteropod, and 12 are heteropods: 1 in the family Pterotracheidae (shell-less), 2 in the family Carinariidae (partially shelled), and 9 in the family Atlantidae (shelled). Well-preserved holoplanktonic gastropods were found in all samples analyzed, except at 122.322–123.323 and 130.490–131.545 mbsf in Hole U1394B. The high volume of volcanic material in these samples suggests that they represent resedimented material and, thus, holoplanktonic remains within them were fragmented beyond identification.

No species datums (first occurrences or last occurrences) were identified during the geological period studied. Therefore, all (extant) holoplanktonic gastropod species encountered during this study have a fossil record of at least 300 ky.

Dominant species and their environmental requirements

At all sites analyzed, there is a single dominant species, Heliconoides inflatus, that constitutes between 3% and 65% (average = 28%) in Holes U1394A and U1394B, 13% and 75% (average = 48%) in Hole U1395B, and 10% and 68% (average = 40%) at Site CAR-MON 2 (Figs. F3, F4, F5). H. inflatus is a warm-water cosmopolitan species (Bé and Gilmer, 1977) that tolerates a wide range of water temperatures (14°C–28°C) (Table T1). This species is found in all samples analyzed, indicating that the water offshore of Montserrat was 14°C–28°C at all times during the last 300 ky.

Five further species (Atlanta selvagensis, Gleba cordata, Limacina bulimoides, Limacina trochiformis and Styliola subula) were found to be abundant across all sites. There are also a number of species that are abundant at only one or two of the sites. This variation in species abundance across a relatively small area of ocean is probably caused by the patchy, swarming nature of holoplanktonic gastropods (Lalli and Gilmer, 1989; Mathew et al., 1990). These species (Table T1) all indicate subtropical to tropical warm waters (Bé and Gilmer, 1977), which supports published data for the water temperatures of this region over the last 300 ky (Schmidt et al., 2006; Foster, 2008).

Species changes with global temperature

The ratio of oxygen isotopes ¹⁶O and ¹⁸O measured from the calcium carbonate of foraminifers are used as a proxy for the global ice volume and general trends in global temperature. In ocean waters, a high proportion of lighter ¹⁶O, which preferentially evaporates from ocean waters, indicates that ¹⁶O is being returned to the ocean through precipitation. However, a lower proportion of ¹⁶O indicates that following evaporation ¹⁶O is locked up in ice and not returned to the ocean, indicating a global cooling. This chemical signature becomes incorporated into the shells of marine organisms and is measured using geochemical techniques (Coussens et al., 2015).

The abundance of all holoplanktonic gastropod species was correlated (using a Pearsons correlation in the PAST statistics package) to the corresponding oxygen isotope data throughout the study interval of each site (Tables T2, T3, T4) to identify any trends in species composition with global temperature changes. A total of 11 species were found to show a significant correlation to the oxygen isotope record at Site U1394, 6 at Site U1395, and 13 at Site CAR-MON 2. However, only one of the correlations (A. peronii group) was reproducible across all three sites. Therefore, although holoplanktonic gastropod species are clearly influenced by global temperature changes, they are not closely associated with small changes in water temperature, and the correlations at singular sites are likely to be coincidental.

The abundance of the warm-water planktonic foraminifer Globorotalia menardii was used as a local indicator of temperature (Le Friant et al., 2008; Coussens et al., 2015). A total of 5 species were found to show a significant correlation to the abundance of G. menardii at Site U1394, 9 at Site U1395, and 15 at Site CAR-MON 2 (Tables T2, T3, T4). However, none of the correlations were reproducible across all three sites. This supports the finding that changes in species abundances are influenced by temperature but not driven by it. This is likely to be because many of the holoplanktonic gastropod species have a wide temperature tolerance relative to the small changes in Caribbean Sea temperature (Bé and Gilmer, 1977) and are thus not greatly influenced by these changes.

Species associations

The abundances of holoplanktonic gastropod species were correlated against each other to highlight species associations (Tables T2, T3, T4). Eight species associations were reproducible across all three sites. Four associations show a positive correlation and four show a negative correlation. Positive correlations suggest that the two associated species have the same environmental requirements and can improve our understanding of some poorly understood species. For example, we do not know the environmental preferences of the atlantid heteropod species Firoloida desmarestia. However, it shows a reproducible positive correlation to the relatively well studied pteropod species Creseis chierchiae, suggesting that they are influenced by similar conditions.

Five of the eight species associations are between a heteropod species and a pteropod species. This may indicate predator-prey relationships because heteropods are carnivorous and are known to prey upon species of pteropod (Richter, 1982; Newman, 1990). However, in four of the five associations, the correlation is negative, indicating that when the pteropod species is abundant the heteropod species is rare. This is the opposite trend that would be expected from a predator-prey relationship, where abundant pteropods would be accompanied by abundant heteropods. This relationship, therefore, suggests that heteropods are feeding on something that is in competition with the pteropods for food but does not leave fossil remains (e.g., copepods). When the abundance of this prey is high, the number of heteropods increases; however, competition for pteropod food is also high, so pteropods numbers are low. When the abundance of the alternative prey is low, heteropod numbers are also low, but the food available to pteropods is increased, so pteropod numbers also increase. This reveals that pteropods are not the favored prey for some species of heteropod.

Only a single, positive species association was found between two species of heteropod (A. plana and A. selvagensis). This supports recent research, which found that species of atlantid often exhibited species specific seasonal increases in abundance to avoid prey competition (Lemus-Santana et al., 2014). Although seasonal trends are unlikely to be detected in this relatively low resolution data, this behavior may partially explain why heteropod species are not generally found to be abundant at the same time.

The data suggest there are two particularly important species and one key species that show reproducible correlations to multiple other species. Creseis clava and H. inflatus correlate to two species and A. plana correlates to four species (including C. clava). The data do not indicate how these species are important to the ecosystem offshore of Montserrat, as they are all present at different abundances.

Top of page   |    Previous   |    Next